Half the world cannot drink a glass of milk without cramping up. But help may be on the way: Scientists report in next month's issue of Nature Medicine that rats unable to digest lactose, a sugar in dairy foods, are cured by a pill that stitches new genes into the cells of the gut. If the technique works in humans, lactose-intolerant people may one day be able to pop a "gene pill" that offers long-lasting treatment for the affliction.

Lactose-intolerant people suffer from stomach pain and diarrhea when they drink milk or eat dairy products because their intestines don't have enough of the enzyme lactase to break down lactose. Sometimes the symptoms are so severe that people avoid dairy products altogether, which can lead to malnutrition and osteoporosis. Tablets containing the bacterial version of lactase can aid digestion, but they must be taken every time dairy products are consumed. To supply lactase over the long haul, Matthew During and his colleagues at Jefferson Medical College in Philadelphia devised a strategy for incorporating the bacterial lactase gene into intestinal cells.

First they smuggled the gene aboard a disabled version of the adeno-associated virus, which thrives in the guts of rats and humans. The team then tube-fed the virus to lactose-intolerant rats. After just a single dose, rat intestinal cells pumped out bacterial lactase for up to 6 months--showing that the gut cells had inserted the gene into their DNA and were using it to manufacture the enzyme. Seven of the rats maintained their weight when fed a lactose-only diet for 2 weeks, while seven other lactose-intolerant rats without the lactase began to waste away on the same diet.

The gene therapy is "worth looking into" as a potential treatment for lactose intolerance in people, says molecular biologist Stephen Krasinski of the Tufts University School of Nutritional Science and Policy in Medford, Massachusetts. Moreover, the results may transcend the bloating and discomfort of lactose intolerance: The technique could serve as a way to transform the gut lining into a factory for replacing missing proteins in other genetic disorders such as hemophilia, says pulmonologist Eric Alton of the Imperial College School of Medicine in London.